How can get grid fields from Channel entry on Custom Addon?

Im Have custom Addon called Cart, they process ajax request with product ID, check if exist a Channel Entry by user ID, and check if exist the product ID in the grid

im Have Channel Called “Cart” This channel have products Grid This grid have price field(integer) and RelationShip field to Products Channel(relation one product to many Cart

im need get an array or objet with price and product ID, like this

$cart=array(
   cart_id=x,
   products=array(
           product_id=x,
           price=xxxxx
   )
)

ym need know how can get all rows from a grid field with Model service or legacy query